1. Field of the Invention
The present invention relates to a novel welding apparatus which imparts a bead having low penetration in a overlaying on a base metal. More particularly, it relates to a welding apparatus which imparts a smooth bead having low penetration and excellent affinity.
2. Description of the Prior Art
It is especially effective to use a reverse polarity soft plasma arc source in the operation of the present invention. Accordingly, the embodiment using a reverse polarity soft plasma torch will be illustrated though it is not limited to the embodiment.
When non-consumable electrode type arc sources such as the conventional TIG arc sources are used in overlaying operation, a penetration having at least 700 to 1000.mu. has been caused and a convex bead shape is formed even though the heating condition is controlled in the best condition.
FIG. 1 shows a schematic view of the conventional overlaying operation and a cross section view as a result of the operation.
The reference numeral (1) designates a tungsten electrode; (2) designates a shield nozzle; (3) designates a welding arc; (4) designates a welding rod; (5) designates a base metal; (6) designates a surface oxide film of the base metal (5); (7) designates a drooping characteristic DC welding power source in straight polarity; (8) designates an overlayed bead; (9) designates an under-cut and (10) designates a crack.
In the conventional overlaying operation, it is clearly disadvantageous to form the convex overlayed bead (8). However, when the dilution of the welded metal is increased by the increase of penetration of bead, the characteristic required for the overlayed metal is modified. When it is a combination of materials for forming intermetallic compound or segregated compound, cracks (10) are caused. When the welding operation is applied on the surface of the base metal having an oxide film, the under-cut (9) is caused in some welding conditions. These are further disadvantages in the conventional overlaying operation.
On the contrary, the shape of overlayed bead can be improved by using the reverse polarity soft plasma arc source.
Referring to FIG. 2, the fact will be illustrated. In FIG. 2, the reference numeral (11) designates a water cooled copper electrode; (12) designates a plasma nozzle; (13) designates a plasma gas; (14) designates a shield gas; (15) designates a soft arc; (16) designates a plasma jet flame; (17) designates cathode spots and (18) designates a power supply for welding which feeds the power in reverse polarity to the soft arc source (15).
The reverse polarity arc forms cathode spots (17) on a material having small work function such as the oxide film (6) on the surface of the base metal (5) and removes these impurities by the cleaning action to expose the bare surface of the base metal (5).
In comparison with the straight polarity arc, it has the characteristic for imparting wide heating distribution whereby the overlayed bead (8) having relatively high affinity can be formed because of the heat source characteristics.
However, a relatively large penetration is formed in the base metal (5) because of large quantity of heat and large blowing force of the plasma jet flame (16) of the arc (15) whereby the above-mentioned disadvantages may be caused.